Some navigation systems are based on signals received from orbiting satellites, such as the Global Positioning System (GPS) satellites. Performance of such systems degrades if the receiver does not have a direct line-of-sight to the orbiting satellites. Performance of such satellite-based systems degrades if the signal from the satellite is blocked such as by buildings, mountains, etc.
Navigation assistance devices are used in concert with the satellite based systems to provide continued navigation capability when satellite visibility is poor due to the absence of a line-of-sight to the satellite. For example, gyroscopes and accelerometers can be used to help determine displacement or relative position. The combination of gyroscopes and accelerometers can provide heading rate measurements which provide information about the rate of change of the heading (e.g., a vehicle making a turn on a road).
In practice, however, noise is present in both the accelerometer and gyroscope outputs. Such noise unfortunately limits the amount of time a reliable position estimate can be provided without the need for recalibration, for example, via satellite signals. The position estimate's “drift” from the true position is mainly caused by errors in attitude computation which in turn is based on the gyroscope's outputs.
Generally low cost gyroscopes and accelerometers are based on Micro-electrical mechanical sensors (MEMS) technology. In current MEMS-based inertial navigation systems, errors in the attitude are the limiting factor for the amount of dead-reckoning that can be performed.